1. Field of the Invention
The present invention relates to a method of manufacturing a transparent high density ceramic material containing oxides of Pb, La, Zr and Ti (hereinafter referred to as PLZT), and in particular to a manufacturing method which includes hot isostatic pressing of a sintered PLZT product for a short period of time.
2. Description of the Prior Art
PLZT has been used for various optical devices depending on the relative content of Pb, La, Zr and Ti, oxides for example, optical modulation devices which utilize the Pockels effect, optical shutters and optical switches which utilize the Kerr effect, image memory devices which utilize the memory effect, as well as optical shutters which make use of the electro-optical scattering effect, image memory devices which makes use of the surface electrostriction effect, optical memory devices which make use of the photochromic effect, image memory devices which make use of an optical ferroelectric effect, etc.
When PLZT is used as the transparent high density ceramic material, extremely high transparency is required in order to prevents scattering or absorption of light passing through the ceramic material.
Accordingly, it is necessary to reduce pores in the inside of the PLZT and to remove excess Pbo, which causes the PLZT yellow and absorb light.
As a method of obtaining PLZT, there is known a so-called monoaxial pressing-type hot pressing method of charging a starting material powder for PLZT ceramic in a die made of aluminum oxide, silicon carbide, graphite, etc; and then pressing the powder by means of a punch made of alumina (U.S. Pat. No. 3,666,666, 1972).
In this method, however, there is a limit for the size of products obtained in view of the strength of die, and punch at high temperature and uniform heating by using an external heater. Further, reaction between the punch or die material and the pressing product is inevitable and, further, the productivity is low.
Meanwhile, as a method of manufacturing piezoelectric ceramics, a hot isostatic pressing method (hereinafter referred to as HIP) using a high temperature and high pressure gas as a pressure medium is known and the applicant has previously proposed a method of manufacturing high density piezo ceramics such as PbTi.sub.3, PbNb.sub.2 O.sub.6, PbTi.sub.x Zr.sub.y O.sub.3 +Nb.sub.2 O.sub.5, etc; by using HIP (Japanese Patent Laid-Open Sho No. 58-182883).
However, even if the above-mentioned method is applied to PLZT, it is difficult to completely remove pores formed at the inside of the PLZT and, particularly, it is impossible to remove large pores, thus making it difficult to attain high transparency and high density as required as an optical device. Further, there is also been known an atmospheric sintering method of sintering in a PbO-oxygen atmosphere as a method of removing large pores formed upon sintering PLZT. However, it is difficult to control the PbO vapor pressure. If the PbO vapor pressure is higher than a predetermined pressure, excess PbO intrudes to the inside of the PLZT, thus turning the PLZT yellow. On the contrary, if the PbO pressure is low, the invasion of PbO can be prevented; however pores in the inside of the PLZT can not be removed, and thus the PLZT will not be stable.
Furthermore, even a combination of the above-mentioned sintering method with HIP can not stably obtain transparent; high density ceramic material having the required high transparency and freedom from pores.
In order to overcome such problems, the present inventor has already proposed that a transparent high density ceramic material having high transparency and freedom from pores can be obtained by sintering PLZT in a vacuum up to 90% of the theoretical density and, further, applying HIP processing (Japanese Patent Laid-Open Sho No. 62-105955).
Further, as the usage of PLZT has expanded the demand for a further improvement in transparency and freedom from pores has increased, has the desire to find a method of manufacturing the same with a good productivity.
Heretofore, sintering of PLZT has required a long time for obtaining the desired high transparency and, for example, it require more than 60 hours for sintering under a normal pressure, requires more than 16 hours for a hot pressing method (hereinafter referred as HP) conducted in an oxygen or reduced-pressure atmosphere, or more than 60 hours even in a multi-step sintering process comprising HP and sintering at a normal pressure or pressure reduction and normal temperature sintering.
Further, in the technique proposed by the inventor described above, sintering conditions and HIP processing conditions, etc; are also selected depending on the PLZT compositions. However, for satisfying high light transmittance, it requires at least more than 10 hours in total, that is, more than 5 hours for sintering at a normal pressure or in vacuum and more than 5 hours for HIP and, accordingly, improvement has been demanded also from an economical point of view and operationability.